Conventionally, there is a device such as a joining device 10 in FIG. 12A that joins a thermoplastic resin to a metal member. The joining device 10 melts a part of a thermoplastic resin plate 12 placed on a fixed base 11 and welds the melted part to a metal plate 13 placed on the resin plate 12, to join the resin plate 12 to the metal plate 13.
The joining device 10 has a positive electrode 14 and a negative electrode 15 in a rod shape which are both pressed against an upper face of the metal plate 13, and a DC power supply 16 having a positive terminal connected to the positive electrode 14 and a negative terminal connected to the negative electrode 15. When the resin plate 12 is joined to the metal plate 13 by the joining device 10, the front end of each electrode 14, 15 to which the power supply 16 is connected is brought in contact with the metal plate 13 with a given pressing force. This contact allows a current 17 to flow through the metal plate 13 from the positive electrode 14 to the negative electrode 15. The metal plate 13 is heated with the current 17, to cause a part of the resin plate 12 that is in contact with a metal portion to have a resin meltable temperature so as to be melted as indicated by a reference numeral 12a. The melted portion is referred to as a melted resin portion 12a. 
As shown in FIG. 12B, the melted resin portion 12a has circular portions 12a1 that spread in a substantially circular shape around electrode dents 14a, 15b corresponding to front end positions of the electrodes 14, 15, and a connecting portion 12a2 that connects one circular portion 12a1 with another in a given width. The melted resin portion 12a having this shape is welded to the metal plate 13, and then, the metal plate 13 is cooled so that the melted resin portion 12a is solidified and joined to the metal plate 13. Thus, the resin plate 12 is joined to the metal plate 13. Patent Document 1, for example, discloses a method as this kind of technique.